Indium gallium nitride (InGaN) has become a topic of intense research due to its invaluable optoelectronic properties. This ternary III/V semiconductor is a seemingly ideal material for LEDs due to its direct bandgap that is tunable across the visible range by varying the relative amounts of gallium nitride (GaN) and indium nitride (InN). InGaN is currently used in commercially produced blue and green LEDs, but the material makes poor yellow and red LEDs due to inherent materials quality issues.
Epitaxial growth of nitride semiconductors has been challenging and difficult to understand. This can be attributed to many materials problems: lack of a native substrate, lattice mismatch to common substrates, solid phase immiscibility between GaN and InN, comparatively high vapor pressure of InN, and difference in formation enthalpies of GaN and InN. These problems contribute to material defects, inhomogeneous alloying, and phase separation that affect the film quality and emission characteristics.
Surfactants have emerged in recent decades as a powerful tool for controlling epitaxial growth and achieving more desirable film qualities. Surfactants are active surface species that modify surface free energy, have negligible solubility in the bulk, and low desorption coefficients. During the growth process, surfactants accumulate on the surface, changing the thermodynamics and kinetics of growth. The effects of surfactants have been reported for many different material systems and diverse results have been seen.
One of the first reported results of surfactant-mediated epitaxial growth was the change in surface morphology of films. It was observed that by reducing the free energy of the growth surface with a surfactant, 3D islanding of the film was kinetically inhibited. Much of the literature attributed this to surfactant-modified atomic surface processes such as surface diffusion and adatom step-edge attachment. Zhang et al. observed an interesting change during the lateral epitaxial overgrowth (LEO) of GaN by OMVPE. A change in the dominant growth facets occurred with the addition of Sb. At 1025° C., undoped GaN had predominant, sloped growth planes of {1101}. When Sb was added to the growth process, the predominant growth facets shifted to vertical {1120}. A similar shift in growth facets occurred at a growth temperature of 1075° C. with sloped sloped {1122} facets shifting again to vertical {1120} facets with the addition of Sb to the growth process.
Alloy composition has also been changed by the addition of surfactant. It has been shown that N incorporation in GaAs:N is reduced by Sb, Bi, and Tl surfactants. This was attributed to accumulation of surfactants on the growth surface blocking N adsorption and incorporation. Furthermore, Sb surfactant was reported to change impurity concentrations in GaAs. Zn and In dopant concentrations increased significantly with the addition TESb to the OMVPE growth process, which was explained theoretically by an intriguing dual surfactant effect involving the presence of H on the surface. A more recent study showed an increase in the In composition of InGaN with the addition of Sb to hydride vapor phase epitaxial (HYPE) growth. This was accompanied by a change in the aligning direction of InGaN nanostructures on the surface.
Another effect that has been reported in the literature is a surfactant-induced change in microstructure. One example of this was the change in CuPt—B ordering in GaInP with the addition of Sb. Small amounts of Sb were shown to decrease the amount of CuPt—B ordering making a more homogeneous alloy. However, above a certain threshold concentration, Sb induced a new triple period ordered structure. This was attributed to surfactant-induced changes in surface reconstruction. Other studies showed that increasing Sb concentrations led to an increase in the presence of lateral compositional modulation in GaInP that reduced the low temperature photoluminescence (PL) peak energy.